In crder to produce steels of high quality, it is important to properly control the amount and form of nonmetallic inclusions by reducing the oxygen content in molten steel and separating and removing nonmetallic inclusions suspended in the melt.
Various refining methods have been proposed or commercialized for performing efficient deoxidation and removal of nonmetallic inclusions in the outside of the furnace. Basically, these methods perform preliminary refining in the melting furnace and transfer the melt to an external ladle for effecting the final refining. The present invention is an improvement of this basic method for performing the final stage of refining in equipment outside the melting furnace.
The operating principles and features of several of the conventional methods for performing the final stage of refining in equipment outside the furnace are hereunder described.
(1) Vacuum degassing
This method is most extensively used for refining speciality steels; its operating principle is to cause turbulence in an unkilled or semi-killed molten steel under a high degree of vacuum, causing sufficiently vigorous CO formation to remove hydrogen and oxygen from the melt.
The treated steel has very low hydrogen and oxygen content and fairly small amounts of nonmetallic inclusions. However, in this method, the slag layer must be eliminated in order to expose the melt directly to the vacuum, and the particles of nonmetallic inclusions suspended in the melt are not adsorbed on the slag and hence cannot be completely eliminated from the melt. Another problem with this method is that in order to create a high degree of vacuum (.perspectiveto.1 Torr), a large-capacity steam ejector must be used, which only results in an extremely high energy cost.
(2) The ladle furnace method
This method is principally designed for removal of oxygen and nonmetallic inclusions; it uses a ladle having a construction similar to that of an Heroult electric furnace and supplies a carbide slag during the refining period. For accelerated reducing reaction, gas bubbling is effected by blowing an inert gas into the ladle from the bottom in an amount that will not instabilize the arc.
This method ensures the production of steels having the necessary high quality, but it has two serious defects: firstly, it requires a considerably high financial investment; and secondly, because of low reaction rates, a prolonged treatment is necessay and this causes an appreciably increased operating cost including such factors as electricity for heating, refractories and electrode rods.
(3) Gas bubbling
The principal object of this method is to provide a uniform temperature distribution and remove any nonmetallic inclusions; the operating principle is to blow an inert gas into an already killed molten stell through a gas-permeable refractory, causing boil to an extent sufficient to cause the suspended nonmetallic inclusions to be adsorbed on the slag for removal from the melt.
This method involves simple procedures and requires low operating costs, but is not capable of achieving satisfactory deoxidation and removal of non-metallic inclusions. Two primary reasons are: the bubbles of inert gas blown into the melt are not capable of inducing as strong CO boil as is caused by the vacuum degassing method (1); and the molten steel is oxidized by the ambient air.
(4) Ca alloy blowing
The three objectives of this method are deoxidation, desulfurization and removal of nonmetallic inclusions, and the operating principle is to blow a Ca alloy powder as carried by an inert gas directly into a molten steel through a refractory pipe while the surface of the melt is covered with a non-oxidizing basic slag.
This method provides steels of high quality with high reaction rates and its capital cost is not very high. However, the use of large volumes of expensive Ca alloys and argon gas not only increases the operating cost but also makes this method unfit for those types of steel which should not contain Ca or Al.
The conventional methods described above have their own merits and demerits and steels of high quality cannot be obtained without increasing either capital or operating costs.
The conditions that ensure effective removal of oxygen and nonmetallic inclusions from molten steel can be summarized as follows:
(a) The melt must be subjected to the proper degree of preliminary refining depending on the refining method, working period and the desired level of refining;
(b) In order to increase the rate of deoxidation and removal of nonmetallic inclusions, agitation of the melt is essential and CO boil as strong as that which is caused by vacuum degassing is desirable;
(c) In order to remove nonmetallic inclusions by adsorption, the melt should be covered with a non-oxidizing slag which should be basic if desulfurization and prevention of resulfurization are also to be realized;
(d) The melt and slag should be perfectly protected against oxidation during the refining period and the FeO content in the slag is desirably not more than 1%.
The present inventors previously made close observation of the effects of ambient pressure on the phenomenon of boiling that occurs in the gas bubbling method and discovered the following important facts on the basis of the analyses of the boiling reaction. Basically, the inventors found that effective deoxidation and removal of nonmetallic inclusions can be realized by properly controlling such factors as the initial conditions of the melt, slag composition, its properties, intensity of bubbling and the ambient pressure (of the atmosphere in the ladle). The conditions to be met are: (1) the gas bubbling method is used as the basic approach; (2) this method is operated at very low pressure close to vacuum so as to induce CO boil which is as strong as that caused by degassing in vacuum and to ensure a nonoxidizing atmosphere; (3) for effective removal of nonmetallic inclusions, a semi-killed molten steel is subjected to boiling treatment in the presence of a proper slag; and (4) in order to significantly reduce the operating cost, the necessary minimum degree of vacuum is to be obtained by an inexpensive vacuum pump, for example, a water-sealed vacuum pump. An invention has already been accomplished on the basis of this approach and a patent was applied for it under Japanese Patent Application No. 75574/1981 (Unexamined Published Japanese Patent Application No. 192214/1982).